This invention relates to a chip-type solid electrolytic capacitor and a method of manufacturing the same, and more particularly to a noble structure of an anode terminal thereof.
Japanese Utility Model Publication Sho62-14673 issued on Apr. 15, 1987 discloses a capacitor structure which uses a simplified resin coating instead of a transfer-molded resin package and which replaces conventional external metal plate terminals or metal caps with coated electrodes in order to reduce the size of the capacitor and to increase volumetric efficiency thereof. An anode coated electrode of this prior art capacitor comprises three conductive layers formed on a resin layer coated around an anode wire. More specifically, a silver paste layer, an electroless-plated layer and a solder layer are formed on the resin layer consecutively. However, since the silver paste layer is used as the undercoating of the electroless-plated layer, silver migration tends to occur in a humid environment and results in a large leakage current. Furthermore, there is a significant difference in thermal expansion coefficients between the silver paste and valve-metals such as tantalum used to form the anode lead wire and anode body. For this reason, poor contact develops between the silver paste layer and the anode lead wires due to temperature changes in manufacture and packaging processes, thereby causing poor electric connection between the plated layers and the anode lead wires. This in turn increases dielectric loss to cause difficulties in the production of solid electrolytic capacitors of high quality and reliability. To minimize this problem, the contact region between the anode wire and the electroless-plate layer should as large as possible, making it difficult to reduce the length of the external portion of the anode wire.
Moreover, if the viscosity of the silver paste is increased at the time of coating in order to apply the paste uniformly over the surface, the thickness of the silver paste layer surrounding the anode lead wire is much larger than that at the peripheral region. The length of portion of the the anode lead wire implanted within the coated anode electrode layer is also then larger than necessary. The contour of the anode coated terminal becomes conical compared to the relatively flatly contoured cathode terminal as a result. Such a large difference in the contours of the electrodes induces a so called Tomb-stone phenomena or Manhattan phenomena during the reflow-soldering process to mount the chip-type capacitor on a printed wire board.